Trendelenberg mechanism

ABSTRACT

A Trendelenberg mechanism for variable height beds of the kind having legs telescopically extensible from and retractable into the corner posts thereof by means of flexible cables extending from the legs to a cable-hauling mechanism, in which the cables are connected to the cable-hauling mechanism in pairs each respectively associated with one of the ends of the bed, and in which one of the cable pairs is disconnectible from the cablehauling mechanism at either of the fully raised or fully lowered positions of the bed and retained at such position while the cable-hauling mechanism is operated to lower or raise the other end of the bed, respectively, thereby to tilt the bed longitudinally in either direction, i.e., to place the occupant in the &#39;&#39;&#39;&#39;head-down&#39;&#39;&#39;&#39; or Trendelenberg position, or in the &#39;&#39;&#39;&#39;headup&#39;&#39;&#39;&#39; or reverse Trendelenberg position.

United States Patent Licina et a1.

[ Mar. 19, 1974 1 TRENDELENBERG MECHANISM Inventors: Charles Licina, Thornton, 111.;

Robert E. Miller, Dyer; Nathaniel H. Taylor, Highland, both of Ind.

Assignee: Simmons Company, New York,

Filed: Mar. 6, 1972 Appl. No.: 231,916

US. Cl. 5/63, 5/64 Int. Cl A61g 7/10, A470 3/32 Field of Search 5/60, 62, 63, 64, 86

References Cited UNITED STATES PATENTS 9/1965 Nordmark et a1. 5/63 11/1965 Black et a1 5/63 5/1964 Huntress et a1 5/63 Primary Examiner-Casmir A. Nunberg Attorney, Agent, or FirmFitch, Even, T abin & Luedeka [57] ABSTRACT A Trendelenberg mechanism for variable height beds of the kind having legs telescopically extensible from and retractable into the corner posts thereof by means of flexible cables extending from the legs to a cablehauling mechanism, in which the cables are connected to the cable-hauling mechanism in pairs each respectively associated with one of the ends of the bed, and

5 Claims, 7 Drawing Figures PATENTEBIIAR 1 9 1974 'SHEU 1 OF ,3

PAIENIEB m 19 1974 men a [If 3 I i M 95 68 24 TRENDELENBERG MECHANISM This invention relates to hospital beds of the kind that are variable in height, and particularly to a bed of that kind in which the elevating mechanism can also be operated to tilt the entire bed longitudinally to place the patient in a head down or head-up position.

The object of the invention is to provide in a cable operated elevating mechanism an inexpensive Trendelenberg adjustment by means of which the cables attached to the four-telescopic legs of the bed may be operated in unison to cause the bed to ascend or descend uniformly, or may be operated selectively in pairs to achieve the head-down, Trendelenberg or shock position, or conversely, the head-up, reverse Trendelenberg, or drainage position of the patient.

It is a more specific object of this invention to provide a cable operated mechanism capable of achieving the aforementioned positions by providing for the disconnection of the cables for one end of the bed from a common cable-hauling drive unit when the bed is either fully raised or fully lowered, and for the retention of that one bed end in such position while the drive unit is operated to lower or to raise, respectively, the opposite end of the bed.

A further object of the invention is to provide a simple mechanical device for securing the disconnected cable hauling mechanism associated with the one bed end in the fully retracted position, i.e., with the associated bed end fully raised, to insure against inadvertent dropping of the raised bed end.

A still further object of the invention is to provide a simple unitary. control mechanism effective by the same control movement to disconnect the drive unit from the aforementioned one bed end at either the fully raised or fully lowered positions of the bed.

The invention will be better understood and the attainment of the foregoing objects appreciated from the following description taken in conjunction with the ac companying drawings in which:

FIG. 1 is a series of four cartoons showing the bed in side elevation in its fully lowered, fully raised, headdown or Trendelenberg, and head-up or reverse Trendelenberg positions; FIG. 2 is a partial assembly view of the cableoperated elevating mechanism with the portions of the bed not essential to the elevating mechanismv broken away to facilitate the orientation of certain details within the overall assembly;

FIG. 3 is a fragmentary plan view of the cable hauling mechanism, particularly of the mechanism for disconnecting one pair of cables from the drive unit and for their retention at the position corresponding to the fully-raised position of their associated bed end;

FIG. 4 is a fragmentary side elevation corresponding to FIG. 3, also partially broken away to emphasize certain details;

FIG. 5 is a fragmentary end elevation of the control mechanism for the selective disconnection of the aforementioned disconnectable cables;

FIG. 6 is a fragmentary enlargement of the cable disconnecting mechanism shown at the left-hand end of FIG. 3; and

FIG. 7 is a corresponding side elevation of the same, showing the two positions of the control mechanism by means of which the aforementioned disconnectable pair of cables is separated from the drive unit.

A quick summary of the mechanism and its operation may be had from FIG. 2, which reveals that the drive unit comprises a screw 10 aligned with the longitudinal center line of the bed, and engaged in a nut 12 at one end of a long draw bar assembly 14 axially aligned with the screw. The head end of the bed is at the left of FIG. 2, and the foot end at the right. The drive screw is located at the foot end of the bed so that the motor and switch gear (not shown), if the bed be electrically operated, are remote from the area of oxygen administration, and so that the screw may be conveniently turned by a crank at the foot of the bed, if operated manually.

The draw bar assembly 14 comprises a pair of paral-.

lel metal straps 16 secured in flanking relation to the drive nut 12 at one end, and at their other to a cross head 18 to which the cables 20 associated with the foot end of the bed are attached. The draw bar assembly 14 continues toward the head of the bed as a single rod 22 of solid circular cross section anchored in the crosshead 18 that draws the foot end cables, and passing freely through a second crosshead 24 attached to the head end cables 26. Immediately headward of the head end cross head 24, the draw bar 22 passes through a ball clutch 28, the housing of which abuts the back side of the cross head 24.

A controlrod 30 at the foot end of the bed, when pulled upwardly by a knob 32 at its upper end, is effective through a bell crank 34 to draw a longitudinal pull rod 36 toward the foot of the bed. The extreme opposite or headward end of the rod 36 is curled about the operating collar of the ball clutch 28, and when so moved when the bed is in the fully lowered position illustrated in FIG. 2, serves to disconnect the ball clutch 28 from the draw bar 22 and to permit the draw bar 22 to pass freely through the head end cross head 24 leaving it behind, i.e., with the cables 26 of the head end fuly extended and thus with the telescoped legs at the head end of the bed fully retracted.

If power is applied to elevate the bed from its extreme low position with the control knob 32 pulled up, only the foot end cables 20 are hauled, the retractable legs to which they are attached are thus extended from their corner posts, and the bed is placed in position c of FIG. 1. When the drive screw 10 is subsequently reversed, the foot end of the bed descends by gravity as the draw bar assembly 14 moves toward the head of the bed, where itbecomes automatically recoupled with the ball clutch 28 that backs up the crosshead 24 for the head end cables 26. If power be again applied to elevate the. bed, both cable crossheads 24 and 18 are drawn toward the foot of the bed, all cables retracted, all legs thereby extended from their corner posts, and

the bed may be fully elevated to position b of FIG. 1.

With the bed in position b, and the control knob 32 once more lifted, a double hook 38, pivoted on the bed frame, is positioned to grasp the sideward guiding extensions of the head end cable crosshead 24. When the power unit is operated, therefore, to send the draw bar assembly 14 back toward the head of the bed, i.e., to lower the bed, the'head end crosshead 24 is securely retained in the hook 38 to retain the head end of the bed in the nearly fully raised position while the foot end descends. If descending motion is continued to its limit, the bed achieves position d of FIG. 1.

The aforementioned operations are accomplished by mechanism now described in detail by reference to FIGS. 3 to 7 inclusive in association with the assembly of FIG. 2.

The structural underframing of the bed is suggested rather than fully illustrated in FIG. 2, but may be readily visualized from the illustration,.together with the cartoons of FIG. 1, as comprising a pair of facing channel rails 40 extending longitudinally of the bed and spaced apart a distance considerably less than the overall width of the bed. These side rails 40 are joined intermediate their ends by a pair of cross rails 42 and 44, also of channel shape, and are connected at their ends to the cross rails of the end frames 46 by means of suitable reinforcing gusset plates 48.

Each end frame comprises a pair of opposed corner posts 50 connected by the cross rail 46, of some depth, to which the longitudinal side rails of the underframe are secured, and as indicated in FIGS. 1 and 2, each corner post 50 houses a telescoped retractable leg 52. The bed is raised by extending the retractable legs 52 from the corner posts, and this in turn is accomplished by the elevating cables 20 and 26, the far end of each of which is staked by means of a swaged, offset fitting in a hole in the wall of the extensible leg 52, which is preferably tubular.

The portions of each elevating cable 26 that is housed within a corner post 50 passes downwardly between the outside of the retractable leg and the inside of the hollow corner post, passes around a sheave 54 and is directed to one of a pair of sheaves S6 in the cross rail of the end frame, from whence the cable 26 proceeds to its connection with the cable hauling crosshead 24. p

The cables 20 for the foot end of the bed are rigged in much the same manner, except that the pair of sheaves 58 at which the cables depart the foot end of the bed are spread farther apart so as to flank the driving motor and transmission, not shown. The foot end cables extend toward the head of the bed passing through holes provided in the cross member 44, and thence about the sheaves 60 in the other cross member 42, reversing their direction to then extend toward the foot of the bed, and being anchored in the fixed crosshead 18 of the draw bar assembly. The foot end cables likewise pass through clearance holes in the head end crosshead 24, which may be disengaged from the draw bar assembly 14 for the execution of the Trendelenberg position, as earlier explained.

As'shown in FIG. 2, the spaced parallel straps 16 I which at one end flank the nut 12 engaged with the drive screw 10 extend through a clearance hole in the web of the cross member 44 of the'underframe', and are welded at their opposite ends to a bar of square cross section which serves as the cross head 18 for the foot end elevating cables 20. At its mid-point, the crosshead 18 is drilled and tapped (FIG. 6) to receive the threaded end of the draw bar 22, which is circular in cross section, coaxial with the drive screw 10 along the foot end cables 20 are retracted to extend the legs from the corner posts of the foot end frame; conversely, when the screw is turned in the reverse direction, moving the draw bar assembly 14 toward the head of the bed, the foot end of the bed descends by its own weight, retracting its legs, which in turn extends the cables.

The disengageable connection between the crosshead 24 for the head end elevating cables 26 is more particularly shown in FIGS. 3, 4, 6 and 7.

It has already been said that the cable crosshead 24 is not fixed in the draw bar assembly 14 but rather that the circular rodportion 22 of the assembly passes freely through the crosshead 24, a clearance hole 62 being provided for that purpose. Flanking the central clearance hole for the draw bar rod 22 are the cable anchores 64 for the head end cables, and flanking them in turn in the cross bar are two additional clearance holes 66 to permit the foot end cables 20 also to pass freely through the crosshead. In its intermediate or midportion the cross head 24 is of square cross section,

but its projecting outer portions 68 are of circular cross section, each milled to provide a flat upper quarter 70 facing the head end of the bed for reasons later noted. The projecting outer portions 68 of the crosshead 24 pass outwardly through guide slots 72 which are formed in the webs of a pair of facing channel members 74 and serve as ways for the crosshead 24. The channels 74 extend longitudinally between the two cross members 42 and 44 of the underframe of the bed, and serve as a housing and support for the cable hauling mechanism.

Still referring to FIGS. 3 to 7 inclusive, there is mounted on the rod portion 22 of the draw bar assem-,

bly 14, between the head end cable crosshead 24 and the underframe cross member 42, the ball clutch 28 which is normally connectedto the rod portion 22 of the draw bar assembly 14 for movement therewith, and which abuts the head end cable crosshead 24 on one side. When the draw bar is moved toward the foot of the bed, the ball clutch 28 after taking up a slight clearance allowed for selective disengagement of the clutch, bears against the back side of the cable cross head 24, likewise moves the crosshead toward the foot of the bed, thus removing the load from the ends of theguide slots 72, and hauling upon the head end elevating cables 26 simultaneously with the hauling of the foot end cables, and causing the bed to rise level. Similarly, when the drive screw 10 is actuated to move the draw bar toward the head of the bed, all cables likewise return in unison until the crosshead 24 re-engages the ends of the guide slots 72 in the websof the facing channels 74.

However, when the bed is in the fully lowered position, the ball clutch- 28 can be operated to disengage itself from the rod portion 22 of the draw bar assembly 14 to permit the draw-bar assembly to move freely toward the foot end ofv the bed, i.e., without hauling upon the head end cable crosshead 24. The latter in that instance maintains its fully lowered position determined by the abutment of its outwardly extending portions 68 with the webs of the facing channel rails at the mounting bracket 78 which extends upwardly from the cross member. These holes provide slide bearings for the rod 30 and may, if desired, be provided with bushings of nylon or the like. The bell crank 34 is pivoted on a pin 80 passing between the upstanding legs of the U-shaped bracket 78.

The bell crank 34 passes through a slot 82 milled in the vertical rod to which it is pivotally connected by means of a pin 84 passing through and anchored in the vertical rod 30 and seated in a short slot 86 in the bell crank. The other arm of the bell crank is pivotally connected by means ofa clevis 88 to the longitudinally extending rod 36 which extends toward the head end of the bed.

At its opposite end, the long pull rod 36 is bent into a hook 90 (FIGS. 4 and 7) curled about and seated within a circumferential groove 92 in the ball clutch actuator 94 through which the rod portion 22 of the draw bar passes. The actuator 94 is cup-shaped on its side facing the clutch proper, and the cup partially houses a spring 96 which encircles the draw bar rod 22 between the actuator 94 and the clutch proper.

The ball clutch proper comprises an inner sleeve 98 having a free slip fit on the draw bar rod 22, which passes through it. The sleeve is provided with a plurality of radial holes, all in the same plane and each forming a cage for a steel bearing ball 100 which provides the locking detent for the clutch. The rod portion 22 of the draw bar is suitably necked down to provide a seat 102 in which to receive the detent balls in locking engagement, the seat being chamfered at both sides to cam the balls radially outwardly to disconnect the rod 22 from the inner sleeve 98 when outward movement of the detent balls 100 is permitted.

Surrounding the inner sleeve 98 is an outer sleeve or cage 104 which has a slip fit with the inner sleeve 98 upon which it is mounted. The interior of the outer sleeve is machined out from one end to provide concentric cylindrical recesses 106 and 108 of different diameters, the lesser 106 serving to confine the detent balls 100 in the necked down portion 102 of the draw bar rod 22, and the greater 108 movable into alignment with the detent balls by the endwise movement of the clutch actuator 94, as illustrated in FIG. 7, to permit the detent balls 100 to move outwardly, and thus to release the draw bar rod 22 when the latter is drawn toward the foot of the bed.

Between the outer sleeve 104 and a split ring 1 emplaced in a groove cut in the outer surface of the inner sleeve 98 of the clutch is a compression spring 112 which normally urges the outer sleeve 104 into its ballretaining position shown in FIG. 6. The clutch disengaging movement, illustrated by broken lines in FIG. 7,

first takes up the clearance between the inner sleeve 98 and the cross head 24 (FIG. 6), as the entire clutch assembly moves to the right, and then compresses the spring 112 as the inner sleeve abuts the crosshead 24 and the outer cage 104 continues to the right. Thespring 112 is maintained in the compressed condition as the detent balls are forced outwardly in their cages in the inner sleeve 98 as the outer cage 104 moves forward, pushed by the actuator 94. When the draw bar rod 22 thereafter departs, the detent balls bear against the full diameter portion of the draw bar rod and prevent the outer cage 104 from returning. Therefore, when the draw bar rod 22 later returns to the position at which the disengagement was made, the detent balls 100, urged by the camming engagement of the step between the concentric cylindrical recesses 106 and 108 of the outer sleeve 104,are returned into locking engagement with the necked down portion 102 of the draw bar rod 22 as the compressed clutch spring 112 returns the outer sleeve 104 to its retaining position. As indicated in FIG. 6, the clutch assembly 28 moves slightly to the left as the ball clutch re-engages the rod 22, thus to provide the necessary clearance for the disengaging movement.

The arrangement is such, therefore, as to require manual lifting of the control knob 30 to disconnect the head end crosshead 24 from the draw bar 22 but to per mit the same to reconnect itself automatically when the draw bar 22 returns to the fully lowered position.

Also, it will be noted that when the draw bar assembly 14 moves to the right as seen in all drawings, i.e., in a direction to elevate the bed, both crossheads 24 and 18 move simultaneously if there has been no manipulation of the Trendelenberg control at the extreme low position. Once removed from the extreme low position, the ball clutch actuator 94 is ineffective to disengage the ball clutch, which, by the movement of the draw bar assembly to the right, or footward, has moved out of the sphere of operation of the clutch actuator, the inner sleeve 98 of the clutch pushing the cross head 24 before it.

In the foregoing manner, manipulation of the Trendelenberg control knob 30 when the bed is raised from its extreme low position serves to disable the head end cables 26, leaving the head end at the extreme low position while the foot end is elevated. This condition is indicated by the position c of FIG. 1.

By manipulation of the same control knob 30 when the bed is lowered from its extreme high position, the bed may be placed in the reverse Trendelenberg or drainage position shown in position d of FIG. 1.

Referring again to FlG. 2, 3 and 4, itwill be noted that the facing channel rails 74 which house the cable hauling mechanism are straddled by the double book 38, which comprises a pair of upwardly open hooks l 14 united as an integral assembly by an upper cross bar 116 which passes over the top of the long control rod 36. The double hook is pivoted on a pintle 118 which passes through aligned holes in the hook portions 114 and in the facing channel rails 74, and is maintained in assembly therewith by a cotter key which prevents its withdrawal. The connecting cross bar 116 that units the two hook portions 114 is notched, as shown particularly in FIGS. 2 and 3 to form a seat for an upstanding stud 120 mounted on the longcontrol rod 36. The stud is drilled diametrically to receive the control rod 36, and drilled and tapped axially to take a set screw by means of which the stud is adjustably positioned on the rod. Referring also to FIG. 4, it will be noted that the double hook 38 is normally urged to the solid position of FIG. 4 by a torsion spring 122 surrounding the pivot pintle 118 and having one of its ends seated in a hole in the adjacent channel web and the other bearing upon the double hook assembly.

The guide slots 72 in the webs of the facing channels 74, through which the outwardly projecting ends 68 of the head end cable crosshead 24 project, are sufficiently long, as is the normal elevation stroke of the draw bar assembly 14, to bring the outwardly projecting ends of the crosshead 24 into the operating zone of the hooks 114. Thus, when the bed is at the fully elevated position, and the Trendelenberg control knob 30 lifted, as before, and the drive unit actuated to return the bed to the low position, the double hook 38, having been moved by the stud 120 to the intercepting broken line position of FIG. 4, serves as a detent to retain the head end cable cross head 24 at the fully raised position while the draw bar assembly, including the ball clutch 28 returns to the fully lowered position, thus placing the bed into the attitude illustrated in position d of FIG.

To insure against a mere casual or temporary engagement of the double hook 38 with the sidewardly projecting ends 68 of the crosshead 24, such as might result from lack of precise alignment of the two hook portions 1 14 of the double hook, the flat 70 is milled in the upper left-hand quarter of each end as viewed in FIG. 4 to provide an unstable center point of contact between the hook point and the crosshead such that the crosshead 24 either seats itself firmly within the hook, or it forces the hook 38 (and the Trendelenberg control knob 30) downwardly. This precaution prevents accidental dropping of the head end of the bed by either assuring firm engagement with the hook or immediate notification of the attendant of his abortive effort, and in any event, the safe operation of the bed.

As in the case of the ball clutch'28, the double hook assembly 38 is likewise self-resetting when the cable hauling mechanism is again returned to the position at which the cable crosshead 24 was disconnected from the draw bar assembly. Specifically, upon return movement of the draw bar assembly 14 to the fully raised position, bringing with it the ball clutch 28, the inner sleeve 98 of the ball clutch re-engages the back edge of the head end cable crosshead 24, moving it slightly to the right from the broken line position in FIG. 4, and freeing the ends 68 of the crosshead 24 from their seating engagement within the hook. With the cross head thus moved slightly toward the foot of the bed from the broken line position shown in FIG. 4, the double hook is returned to its depending full line position of FIG. 4 by the action of the return spring 122. The head end cable crosshead 24 thus remains supported by the ball clutch 28 for its return to the fully lowered position, absent further operation of the Trendelenberg control.

By providing a disengageable connection between one of the cable hauling mechanisms and the drive unit together with a single control mechanism operable to retain the disengaged cablehauling mechanism at either its-fully lowered or its fully raised position while the cable hauling mechanism for the opposite end of the bed continues its operation in the usual fashion, a simple and relatively inexpensive and integrated mechanism is provided for executing the Trendelenberg and the reverse Trendelenberg positions in a hospital bed.

The features of the invention believed new and patentable are set forth in the following claims.

What is claimed is:

l. in an elevating mechanism for a variable height bed having extensible and retractable legs and flexible cables trained from said legs to a common cable hauling mechanism on the frame of the bed, a Trendelenberg adjustment which comprises the connection of said cables to said cable hauling mechanism by separate pairs respectively associated with opposite ends of the bed, the connection of one of said cable pairs being disengageable from the cable-hauling mechanism at either the retracted or the extended positions of the bed legs associated therewith, thereby to tilt the bed longitudinally by subsequent extension or retraction respectively of the legs associated with the other cable pair, and means on the bed frame for carrying the load on said one cable pair when disengaged from said cablehauling mechanism.

2. The subject matter of claim 1 wherein the cablehauling mechanism comprises a screw-actuated draw bar, and the disengageable cable pair is secured to a crosshead movable by said draw bar in ways on said bed frame, and said load carrying means includes the engagement of said crosshead with the bed frame at the one end of said ways corresponding to the retracted position of the associated bed legs, and detent means on said bed frame selectively engageable with said crosshead at the other end of said ways to maintain the extended position of said associated bed legs when the draw bar is actuated to permit the retraction of the legs associated with the other cable pair, and wherein the disengageable connection of the draw bar and crosshead is a clutch normally engaged to transmit the cable load on said crosshead to said draw bar when said bed is elevated.

3. The subject matter of claim 2 wherein the selective engagement of said detent means and the selective disengagement of said normally engaged clutch are effected by a common manually operated control.

4. The subject matter of claim 2 wherein the draw bar comprises in part a rod passing freely through said crosshead and having thereon a ball clutch positioned to engage the crosshead on one side thereof to transmit the cable load to the draw bar, and wherein said detent means comprises a normally disengaged hook member movable on said bed frame to engage said crosshead to retain said disengageable cable pair in full legextending position when the draw bar is subsequently moved to permit the retraction of the legs associated with the other cable pair.

5. The subject matter of claim 2 wherein the ways for the cross head are defined by longitudinal slots in the webs of facing channel members which flank and guard the draw bar, and the crosshead has ends projecting sidewardly through said slots, and said .detent means comprises an upwardly open hook on the outside of each channel member and connected to the other hook by an integral cross member spanning the two hooks, each said hook pivoted to its respective channel member on an axis common to both and raisable to engage the sidewardly projecting ends of the crosshead by a movement of the cross bar longitudinally of the bed, said clutch being disengageable by a longitudinal control movement in the same direction as the engaging movement of the integral cross member of said hooks, and a common control for said clutch and hooks comprising a rod running longitudinally of the bed from said clutch and hooks to the foot of the bed and having means adapted to disengage said clutch and to cause said hooks to engage the projecting ends of said crosshead by movement of said rod in said direction, said rod being movable longitudinally by a control handle at the foot of the bed. 

1. In an elevating mechanism for a variable height bed having extensible and retractable legs and flexible cables trained from said legs to a common cable hauling mechanism on the frame of the bed, a Trendelenberg adjustment which comprises the connection of said cables to said cable - hauling mechanism by separate pairs respectively associated with opposite ends of the bed, the connection of one of said cable pairs being disengageable from the cable-hauling mechanism at either the retracted or the extended positions of the bed legs associated therewith, thereby to tilt the bed longitudinally by subsequent extension or retraction respectively of the legs associated with the other cable pair, and means on the bed frame for carrying the load on said one cable pair when disengaged from said cable-hauling mechanism.
 2. The subject matter of claim 1 wherein the cable-hauling mechanism comprises a screw-actuated draw bar, and the disengageable cable pair is secured to a crosshead movable by said draw bar in ways on said bed frame, and said load carrying means includes the engagement of said crosshead with the bed frame at the one end of said ways corresponding to the retracted position of the associated bed legs, and detent means on said bed frame selectively engageable with said crosshead at the other end of said ways to maintain the extended position of said associated bed legs when the draw bar is actuated to permit the retraction of the legs associated with the other cable pair, and wherein the disengageable connection of the draw bar and crosshead is a clutch normally engaged to transmit the cable load on said crosshead to said draw bar when said bed is elevated.
 3. The subject matter of claim 2 wherein the selective engagement of said detent means and the selective disengagement of said normally engaged clutch are effected by a common manually operated control.
 4. The subject matter of claim 2 wherein the draw bar comprises in part a rod passing freely through said crosshead and having thereon a ball clutch positioned to engage the crosshead on one side thereof to transmit the cable load to the draw bar, and wherein said detent means comprises a normally disengaged hook member movable on said bed frame to engage said crosshead to retain said disengageable cable pair in full leg-extending position when the draw bar is subsequently moved to permit the retraction of the legs associated with the other cable pair.
 5. The subject matter of claim 2 wherein the ways for the cross head are defined by longitudinal slots in the webs of facing channel members which flank and guard the draw bar, and the crosshead has ends projecting sidewardly through said slots, and said detent means comprises an upwardly open hook on the outside of each channel member and connected to the otHer hook by an integral cross member spanning the two hooks, each said hook pivoted to its respective channel member on an axis common to both and raisable to engage the sidewardly projecting ends of the crosshead by a movement of the cross bar longitudinally of the bed, said clutch being disengageable by a longitudinal control movement in the same direction as the engaging movement of the integral cross member of said hooks, and a common control for said clutch and hooks comprising a rod running longitudinally of the bed from said clutch and hooks to the foot of the bed and having means adapted to disengage said clutch and to cause said hooks to engage the projecting ends of said crosshead by movement of said rod in said direction, said rod being movable longitudinally by a control handle at the foot of the bed. 